In this long-winded saga, Spencer (Althorp: The Story of an English House), the Ninth Earl Spencer and brother to the late Diana, Princess of Wales, guides us through the Spencer family's long history. Supposedly begun by Robert Despenser (steward to William the Conqueror) in 1066, the earliest ancestry of the Spencers remains in dispute--but there is no doubt that the family line goes at least as far back as the Middle Ages, when a series of wealthy landowners named John Spencer made a fortune herding sheep. Placing biographical portraits of family members against a carefully researched historical background, Spencer goes into the sort of excruciating detail that will interest only those with the most consuming interest in English aristocracy. There are, however, some compelling sections about those Spencers who raised themselves up through scandalous political scheming. Robert Spencer (1641-1702), the Earl of Sunderland, plotted to unseat King James II because the king was a Catholic, but after the scheme failed the unprincipled Robert converted to Roman Catholicism. Sarah Marlborough, related to the Spencers through marriage, had a long, colorful career of aggressively advancing her family's interests. But Spencer provides disappointingly little insight into the most famous Spencer of all time, Princess Diana. And although ably written and extensively researched, this book doesn't have enough of a narrative thread to keep the pages turning. B&w and color photos not seen by PW. (Sept.)